Assess the level of technical debt and test new features in a sandbox before enabling in production. This best practice would help UC to identify and reduce any technical debt that may be affecting their system performance, such as inefficient code, redundant configurations, or outdated features2. Testing new features in a sandbox would also help UC to ensure that they work as expected and do not introduce any new issues or conflicts with existing functionality3. A sandbox is an isolated copy of a production environment that allows developers and testers to work safely without affecting live data or users4.
https://admin.salesforce.com/blog/2021/tech-debt-what-it-is-and-why-you-should-care

https://developer.salesforce.com/docs/atlas.en-us.222.0.object_reference.meta/api/big_object.htm Big objects are a way to store and manage massive amounts of data on the Salesforce platform12.
Big objects support custom Salesforce Lightning and Visualforce components rather than standard UI elements3.
Big objects do not support picklist fields, so they need to be loaded as text fields into the big object2.
Big objects do not support standard SOQL queries, so they need to use Async SOQL to query data from them2.

https://www.salesforce.com/news/stories/introducing-revenue-cloud/
https://www.salesforce.com/products/cpq/overview/
1. Migrate all historical payment methods from the homegrown system. According to 1, Revenue Cloud helps you manage your billing and payment processes across different channels and systems. To reduce customer impact, you should migrate all historical payment methods from your homegrown system to Revenue Cloud so that you can continue to process payments smoothly and securely for your existing customers.
2. Compare invoices as produced in both systems to ensure customer invokes are as expected. According to 2 and 3, Revenue Cloud allows you to create and manage invoices for your customers based on various factors such as contracts, subscriptions, usage, etc. To reduce customer impact, you should compare invoices as produced in both systems to ensure that they are accurate and consistent with your customer expectations and agreements.
3. Create a release and change management process to incorporate feedback and fix issues. According to 4, following best practices for release and change management can help you improve your revenue performance and customer satisfaction. To reduce customer impact, you should create a release and change management process that allows you to incorporate feedback from your customers, end users, admins, etc., and fix any issues that may arise during or after the go live.

Set up lead nurturing with Marketing Cloud and automate emails through journeys. This recommendation would help UC to send personalized and targeted emails to customers who abandoned their cart, based on their behavior or preferences1. For example, UC could send emails with discounted offers, cross-sell or upsell suggestions, testimonials, or reminders to complete their purchase3. Marketing Cloud journeys can also track customer engagement and measure campaign effectiveness2.
Create an opportunity when a customer clicks a cross-sell or upsell email link. This recommendation would help UC to identify customers who are interested in buying more products or services from them, based on their email interactions1. For example, if a customer clicks on a link that shows similar products to their cart items, UC could create an opportunity in Sales Cloud and assign it to a sales representative for follow-up3. This way, UC could increase their sales revenue and conversion rate by focusing on qualified leads2.
Stop creating leads in Sales Cloud for abandoned carts. This recommendation would help UC to reduce the Sales team's workload and avoid cluttering their system with unqualified leads1. By creating leads only for customers who show interest in buying from UC, UC could streamline their lead management process and improve their lead quality3.